ACTIVATION OF VOLTAGE-INDEPENDENT CA2+ ENTRY BY NORADRENALINE INVOLVES CGMP IN VASCULAR MYOCYTES

Stimulation of portal vein myocytes with noradrenaline (NA) in the pre sence of a voltage-dependent Ca2+ channel blocker, evoked a transient increase in the concentration of free cytosolic Ca2+, due to inositol 1,4,8-trisphosphate mediated Ca2+ release, followed by activation of a Ca2+ entry pathway, Combining patch-clamp and Indo-1 measurements we have tested the effects of various pharmacological agents on this Ca2 entry following NA-induced Ca2+ release in order to determine the mec hanism involved, Only the guanylate cyclase inhibitor LY-83583 specifi cally inhibited the maintained Ca2+ entry during NA stimulation, This inhibition was reversed by dibutyryl cGMP (DB-cGMP) or 8-bromo cGMP, U nder control conditions, addition of DB-cGMP to the external solution was without effect, Thapsigargin and caffeine each depleted the intrac ellular Ca2+ store but did not evoke Ca2+ entry in venous myocytes und er control conditions, However, application of DB-cGMP or NA after Ca2 + store depletion induced by caffeine or thapsigargin caused a rise in [Ca2+](i) by activation of a Ca2+ entry pathway. The effect of cGMP s eems to involve phosphorylation since cGMP-activated protein kinase in hibitors KT-5823 and H-8 blocked the NA-induced Ca2+ entry, Our result s thus suggest that the activation of the voltage-independent Ca2+ ent ry by NA involves an increase in cellular cGMP.